A bizarre disorder that causes people to physically act out their dreams while sleeping is associated with a dramatically increased risk of developing dementia, and Alzheimer's and Parkinson's diseases, according to new research that suggests the sleep disorder may actually be an early symptom of those conditions.

People with REM sleep behavior disorder, a condition caused by the brain's failure to immobilize a person's muscles while they're dreaming, have an estimated 52 percent risk of developing one of those neurological diseases within a dozen years, according to a study published in today's Neurology. Among people without REM sleep behavior disorder, that risk is about 5 percent, according to study author Ron Postuma, an associate researcher in neurology at the Sleep Disorders Center at Sacre-Coeur Hospital in Montreal.

His study followed 93 people with REM sleep behavior disorder for 12 years. After five years, 26 had developed dementia: 14 came down with Parkinson's, seven had Lewy body dementia (which resembles Alzheimer's with features of Parkinson's), four developed Alzheimer's and one suffered atrophy in multiple brain systems.

REM sleep behavior disorder is believed to be caused by a degeneration of the brain stem — similar to the degeneration that occurs in Parkinson's and Lewy body dementia, Postuma says. But people typically aren't diagnosed with these disorders until they have progressed deeper into the brain and produce noticeable symptoms such as tremors and stiffness. But Postuma tells ScientificAmerican.com that REM disorder may indicate "the degenerative process [of dementia] has started but the only symptom they have is this sleep problem."

The association between REM sleep behavior disorder and dementia has been observed before, but this study tracked it over a longer period than previous research, Postuma says. The finding means that one day, doctors may be able to start treating patients for dementia far earlier in the course of their disease, perhaps slowing or stopping it, Postuma says.

The prevalence of REM sleep behavior disorder isn’t known, he adds. It is different from night terrors, when a person wakes up frightened out of a deep, as opposed to REM — or Rapid Eye Movement — sleep. It mostly affects men in their 50s, 60s and 70s, causing them to punch, kick or cry out while in the dream phase of sleep. Normally, only our eyes and respiratory systems can move during REM sleep.

Image © iStockphoto/Kiyoshi Takahase Segundo